1. Field of the Invention
The present invention relates to a compact and two-lens-group zoom lens system having a short back focal distance, which is suitable for a lens shutter camera.
2. Description of the Prior Art
Unlike a zoom lens system for a single lens reflex camera which requires a space for a quick-return mirror behind the photographing lens system, a zoom lens system for a compact camera does not require a long back focal distance. Accordingly, as a zoom lens system for a lens shutter camera in which there is no need to prepare a space for a quick-return mirror, a telephoto-type lens system, which (i) includes a positive front lens group and a negative rear lens group, in this order from the object, and (ii) performs zooming by varying the distance between the front lens group and the rear lens group, is suitable for miniaturization of a compact camera.
One solution for increasing the zoom ratio of such a telephoto-type zoom lens system is to (i) secure the distance between the front lens group and the rear lens group as long as possible at the short focal length extremity, and (ii) increase the amount of change in the distance between the front lens group and the rear lens group.
However, a simple increase of the distance therebetween results in an increase of the diameter of each lens group. This is because the diameter of each lens group has to be made larger in order to secure peripheral illumination. Consequently, demands for miniaturization and weight reduction on the zoom lens system cannot be satisfied.
The present invention provides a miniaturized and low-cost zoom lens system having a zoom ratio of 3 or more; and more specifically, such a zoom lens system is materialized in a two-lens-group zoom lens system which is suitable for miniaturization and simplification of the structure thereof.
As an aspect of the present invention, there is provided a zoom lens system including a positive first lens group, and a negative second lens group, in this order from the object. Zooming is performed by moving the positive first lens group and the negative second lens group in an optical axis direction.
The positive first lens group includes a negative first lens element 1a, a second lens element 1b having an aspherical surface on which the curvature is made smaller toward the periphery thereof, and a positive third lens element 1c, in this order from the object.
The negative second lens group includes a positive fourth lens group 2a and a negative fifth lens group 2b, in this order from the object.
The zoom lens system satisfies the following conditions:
xe2x88x927 less than fT/f2G less than xe2x88x924.5xe2x80x83xe2x80x83(1) 
xe2x88x929 less than fT/f2GN less than xe2x88x927xe2x80x83xe2x80x83(2) 
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity;
f2G designates the focal length of the negative second lens group, and
f2GN designates the focal length of the negative fifth lens group 2b in the negative second lens group.
Note that in the negative second lens group, each of the positive fourth lens group 2a and a negative fifth lens group 2b can be constituted by a single lens element or more than two lens elements, while the positive first lens group includes the three lens elements. However, from the viewpoint of costs, it is preferable that each of the positive fourth lens group 2a and a negative fifth lens group 2b be constituted by a single lens element.
The zoom lens system preferably satisfies the following condition:
4 less than fT/f1G less than 6xe2x80x83xe2x80x83(3) 
wherein
f1G designates the focal length of the positive first lens group.
The zoom lens system can satisfy the following condition:
2 less than fT/f2GP less than 4xe2x80x83xe2x80x83(4) 
wherein
f2GP designates the focal length of the positive fourth lens group 2a in the negative second lens group.
The zoom lens system can further satisfy the following condition:
xe2x88x925 less than fW/r1 less than 0xe2x80x83xe2x80x83(5) 
wherein
fW designates the focal length of the entire zoom lens system at the short focal length extremity, and
r1 designates the radius of curvature of the object-side surface of the negative first lens element la in the positive first lens group.
The zoom lens system preferably satisfies the following condition:
1.75 less than NdL1xe2x80x83xe2x80x83(6) 
wherein
NdL1 designates the refractive index, with respect to the d-line, of the negative first lens element la in the positive first lens group.
The zoom lens system preferably satisfies the following condition:
64 less than xcexddxe2x80x83xe2x80x83(7) 
wherein
xcexdd designates the Abbe number of the positive third lens element 1c in the positive first lens group.
In the positive first lens group, a lens element on which at least one aspherical surface is formed is provided, and the aspherical surface preferably satisfies the following condition:
xe2x88x9240 less than xcex94IASP less than xe2x88x9210xe2x80x83xe2x80x83(8) 
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
In the negative second lens group, a lens element on which at least one aspherical surface is formed is provided, and the aspherical surface preferably satisfies the following condition:
0 less than xcex94VASP less than 0.5xe2x80x83xe2x80x83(9) 
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-262753 (filed on Aug. 31, 2001) which is expressly incorporated herein in its entirety.